Cathode



Jan. 13, 1959' H. BRETT 2,869,031

CATHODE Filed March l, 1955 HERBERT BRETT A TTORNE' Y 2,869,031 CATHODE Herbert Brett, Fair Haven, N. J., assignor to the United States of America as the Army represented by the Secretary of The invention described herein may be manufactured and. used by or for the Government for governmental purposes without the payment of any royalty thereon.

The invention relates to cathodes for use in electronic devices and particularly to such devices wherein relatively high current densities are required and wherein the ow of electrons from the ca-thode is confined to a beam having a predetermined geometric form.

The invention utilizes the broad principles of the hollow cathode type of design to provide an electron beam having relatively high current density and one which is projected outwardly radially 'from a peripheral slot in a cylindrical cathode or the cathode may assume a form in which a narrow beam is radiated from a linear source of electrons of substantial length. The invention also accomplishes complete control over the beam thickness, which at its origin may be only a few one-thousandths of an inch in thickness. i

The emissive portion of the cathode is provided` with an elongated hollow chamber having a quantity of electron emissive material therein. This material, desirably, is coated on the inner walls of the chamber. In the form of the invention to be described hereinafter thechamber is annular in form.

The cathode, desirably, functions as an indirectly heated element, being heated by suitable electric heating means such `as a conventional ilamentary heater.

Electrons which are released from the active material pass outwardly `through a narrow slit extending along the length of the chamber. In a preferred form the slit extends circumferentially completely around an annular chamber. in such a device an electron 'beam is formed which is quite thin in a plane extendingjoutwardly from the slit and in the special construction to be described hereinafter the beam extends radially outward through an arc of 360 thus imparting to the beam a disc-like conformation. Such a beam is very effectively employed in a magnetron having a conventional generally circular anode and wherein the cathode is placed in the center thereof surrounded by an interaction space. The invention is also useful in other types of apparatus such as beam type power ampliiier tubes.

It is a primary object of the invention to provide a long life cathode capable of producing a sustained constant current density electron |beam of substantial power capacity. i

A further object of the invention is to provide an electron beam of controlled conformation.

A still further object of the invention is to provide a cathode having a large capacity reservoir of electron emissive material having a high work function.

A still further object of the invention is to provide a cathode wherein its high work function `surfaces are protected from secondary electronic bombardment.

A further object of the invention is to provide a cathode which has a minimum noise component.

Other objects and features of the invention will more 2,869,031 Patented Jan. 13, 1959 ttl@ 2 fully appear from the following description and will be particularly pointed out in the claims.

To provide a better understanding of the invention a particular embodiment thereof will be described and illustrated in the accompanying drawings wherein Fig. l is a partial cross-sectional view illustrating a preferred form of the invention applied to a magnetron.

Fig. 2 is a plan view showing the anode of the magnetron and the position of the cathode with respect thereto.

As above indicated, the invention provides a cathode of novel construction which produces a beam of electrons which is quite thin in one plane but wide in a plane at right angles to its thin plane. A cathode embracing the principles of the invention may assume a generally linear conformation. That is to say, the position from which the beam is initiated may extend along a substantially straight line. A form of the Vcathode has been chosen for illustration, however, in which the electrons are initially projected along a circular or curved line. Specifically, the ibeam is projected 360 outwardly from a circular slot extending around a cylindrical cathode body. Since the device functions at high efficiency when used in a magnetron it will be presented in connection with this type of apparatus.

The main body of the cathode 5 is generally cylindrical in form and is constructed of. any suitable metal such as tungsten or nickel. The lower end of the cathode is extended and suitably supported in a sealed-off terminal member of conventional construction, not shown. The cathode may, however, be supported in any suitable manner such as by means of separate radial supports connected to opposite ends thereof.

The general structure of the magnetron may be conventional consisting of a circular or ring type anode 6 having a plurality of cavity resonator 7 surrounding the cathode and extending into the annular interaction space between the cathode and the anode. A probe or loop 9 conducts power from the magnetron through a conventional output terminal and suitable input connections 10 and 11 shown diagrammatically provide for supplying input energy to the tube.

The cathode 5 may be hea-ted in any desired manner. As shown herein, its heating element consists cf a helical lamentary heater 12 which is embedded heat conducting refractory material in the conventional manner. The heater 12 surrounds a portion of the `cathode which is of reduced diameter situated adjacent to but outside of the interaction space 8. Heat may be applied to the cathode in other ways such as by forming an axial recess in the body of the cathode within which is received a resistance heating element. The heater should be shielded with a high work function material to prevent stray electrons from escaping from the filament and to insure that heat from the filament will not cause direct emission from the shield.

The emissive portion of the cathode is provided with a cavity or reservoir space 13 which is annular in form extending around the periphery of the cathode body. This chamber may have any suitable cross-sectional shape. A desirable shape for the cavity is circular in cross-section to simulate the structure of the spherical cavity used in a pencil beam type of hollow cathode. The complete chamber as above described would be toroidal in form since it is generated by rotation `of a circular pla-ne about an axis. For practical reasons the complete toroid has been cut in half along a substantially central area extending around the toroid to produce a half toroid with a substantially at outer boundary. The outer wall 14 of the half toroidal chamber is formed by a thin-walled sleeve. The wall 14 may desirably be composed of two metallic sleeve members 16 and 17 fitted tightly over the cathode body and having their adjacent edges spaced to form a slit 15.

The sleeves 16 and 17 are` made of material having a high work function. The lower sleeve desirably extends downward to surround the refractory material within which the Filament is received. Desirably, but not necessarily, an additional cylindrical shield 18 may be provided surrounding the cathode adjacent the filament 12.

The inner surfaces of the cathode 5 are coated with a layer 19 of material having a low work function the emissive material extending to the edges of the slit 15. The manner in which the cathode functions is considered to be similar to that postulated for the hollow cathode of the spherical type which generates a narrow pencil beam of electrons. When the cathode has assumed an emissive temperature as a result of the heating Veffect of the filament 12 and the tube elements are energized with suitable input power, a stable electron cloud is generated within the reservoir space 13 having maximum electron density near its surface. This cloud may have an energy of several hundred volts per square centimeter. The total energy present is computed as the sum of the electrostatic pressure and the pressure due to the kinetic energies of the electrons. Thus, when an outlet such as the slit 15l is formed in the wall of the cavity a copious stream of electrons flow from the slit. This beam of electrons flows into the interaction space 8 in the case of the magnetron shown herein where they become available to supply the output energy for the magnetron.

The coating 19 constitutes a reservoir to mai-ntain an adequate supply of free electrons and inasmuch as the coating 19 is protected by the wall 14 from destructive bombardment, the life of the cathode is greatly prolonged. lt is also a characteristic of the invention that during the life span of the device in which the cathode is used the How of electrons is uniform and undesirable noise which may originate at the cathode is greatly reduced. A factor which contributes to the noise-free operation of the cathode is the shape of the electron beam. Electrons passing from the cathode slit 15 to the anode 6 diverge only to a slight degree as shown in dotted lines in Fig. l. Substantially all 4of the electrons go to the anode while none are permitted to travel laterally into the free space outside of the normal interaction chamber between cathode and anode.

The characteristics pointed out above make the cathode highly desirable for use in a magnetron where current densities are relatively great and where operating conditions are severe. The use of a cathode constructed according to the invention permits operation of a magnetron at peak output for long periods of time without inter- What is claimed is:

1. In a magnetron having a substantially annular anode, including a plurality of cavity resonators disposed to present their output opening inwardly, a cathode disposed in said anode comprising a cylindrical `body of high work function metal, an annular chamber within and extending circumferentially around said cathode, said chamber having a thin outer wall, a coating of low work function electron emissive material on the wall of said chamber, a-nd a slit opening outwardly from said chamber in the plane of said anode and extending circumferentially around said chamber and means to heat the cathode to the emissive temperature of said emissive material.

2. An electronic discharge device having an anode and a cathode, said cathode comprising a body of high work function metal having an annular chamber therein with i a narrow externally opening slit extending peripherally ruption due to loss of emission or changes in power output capacity.V

To facilitate application of the coating 19 the upper portion of the cathode body may be separable from the lower portion as shown at 20. After the coating operation the upper section is pressed tightly into place upon the lower section.

along the chamber, a quantity of low work function material partially filling said chamber, means to heat said low work function material to emissive temperature, said anode being spaced from and in the plane of said slit whereby electrons flowing therefrom under the influence of an electric eld between said anode and cathode will travel in a straight line, said electrons combining to form 'a planar 'beam substantially perpendicular to surface of the cathode adjacent the slit.

3. An electronic discharge device having an anode and a cylindrical cathode, said cathode comprising a body of high work function metal, having an annular chamber within and extending around the periphery of said body, said chamber having a thin outer wall with a narrow slit formed therein to constitute an outwardly opening chan* nel from said chamber and'extending substantially around the periphery of said cathode, a quantity of low work function material partially lling said chamber, means to heat said low work function material to its emissive temperature, said anode being spaced from and in the plane of said slit whereby electrons owing from said slit form a thin planar nondispersive disk like beam.

4. An electronic discharge device according to claim 3 and in which the cross sectional contour of said annular chamber is at least partially concave.

5. An electronic discharge Vdevice according to claim 3 and in which said narrow slit extends completely around the periphery of said cathode.

References Cited in the le of this patent UNITED STATES PATENTS 

